1. Field of the Invention
The present invention relates to integrated circuits, and, in particular, to low-noise amplifiers.
2. Description of the Related Art
As power becomes more important for integrated transceivers, lower power supply voltages will be used. As a result of this trend, many problems surface. When designing low-noise amplifier (LNA) circuits, maintaining linearity in the presence of a large signal is particularly difficult with ever shrinking power supply voltages. In order to maintain linearity, one must allow the bias to provide a constant voltage to the device regardless of the current required to maintain that voltage. For discrete LNAs, this problem has been solved simply by using a large inductor to provide bias from the power supply. This approach, however, is not easily done with integrated circuit LNAs. The primary reason for this difficulty is that large inductors require large amounts of area and are therefore very expensive to make.
FIG. 1 shows a schematic diagram of a prior-art LNA. In this figure, the bias currents are defined by all capital letters and the ac signal currents are defined by lower case letters. The emitter current I.sub.e is the sum of the bias current I.sub.BIAS, the ac signal base current i.sub.b, the average collector current I.sub.C, and the ac signal collector current i.sub.c, where I.sub.C is defined as Beta*I.sub.BIAS, i.sub.c is defined as Beta*I.sub.b, and Beta is defined as the low-frequency current gain.
FIG. 2 shows a graphical representation of the base voltage (in volts) at transistor B11 of FIG. 1 as a function of time (in nanoseconds) for input signals having different amplitudes (i.e., 0.001V, 0.101V, 0.201V, 0.301V, and 0.401V) at a power supply voltage V.sub.CC of 2.7V. As can be seen from FIG. 2, as the input signal level increases, the average base voltage decreases. This is caused by a limited I.sub.BIAS. The average base voltage is a function of the average available current at the base. As the signal level increases, the result is a demand for more average bias current I.sub.BIAS. This demand for a higher I.sub.BIAS is due to the requirement that transistor B11 must maintain a constant gain. To maintain a constant gain for an increasing input signal, transistor B11 must have an increasing average collector current I.sub.C. Since the average base current is directly proportional to the average collector current, more I.sub.BIAS is required. As a result, the constant current source is unable to meet the demand of average base current for large input signals, the steady-state base voltage decreases, and the device experiences premature gain compression.
In addition to this gain compression problem, circuits are often required to operate from low supply voltages. Typically, a certain minimum voltage is required in order to bias a circuit for proper operation. As supply voltage decreases, the available voltage approaches or, in some cases, drops below the minimum voltage needed to bias the circuit.
Furthermore, for LNA circuits, noise immunity is very important. If noise from the bias circuit enters into the LNA the performance of the LNA is compromised. Without due consideration and proper circuit techniques, attempts to improve bias problems impact the primary function of the LNA.
FIG. 3 shows a schematic diagram of a traditional LNA bias implementation that illustrates a simple solution that will increase linearity a small amount. This is a simple voltage drive circuit similar to an inductor with the exception of not having a low output impedance at low frequencies. The circuit suffers from the problem of having a high output impedance at low frequencies, which causes linearity problems as previously discussed by K. L. Fong and R. G. Meyer, "High-Frequency Nonlinearity Analysis of Common-Emitter and Differential-Pair Transconductance Stages," IEEE Journal of Solid State Circuits, April 1998, pp. 548-555, and by J. Durec, "An Integrated Silicon Bipolar Receiver Subsystem for 900 MHZ ISM Band Application," 1997 BCTM Conference Proceedings, October 1997, pp. 57-60. Moreover, large values of resistance R32 are required to provide high RF impedances to isolate the LNA from the bias circuit which provides noise immunity from the bias circuits. Because R32 must be large, a subsequent large voltage drop occurs across R32, which decreases the base-to-emitter voltage VBE of transistor B31 and therefore results in premature compression.
FIG. 4 shows a schematic diagram illustrating the basic concept for other prior-art amplifiers, which have good linearity. See J. Durec, "An Integrated Silicon Bipolar Receiver Subsystem for 900 MHz ISM Band Application," 1997 BCTM Conference Proceedings, October 1997, pp. 57-60, and S. Wong, S. Luo, and L. Hadley, "A 2.7-5.5V 0.2-1W BiCMOS RF Driver Amplifier IC with Closed-loop Power Control and Biasing," 1998 ISSCC Digest of Technical Papers, Feb. 1998, pp. 52-53. In these solutions, there is no filtering to provide noise immunity from the bias circuits. Also, there is nothing addressing headroom problems. The typical output impedance of a low output impedance operational amplifier can only maintain output current to within one V.sub.be from the positive voltage supply. In this configuration, the resistor R.sub.set is used to determine the amount of power the device can handle. By selecting different values of R.sub.set, different amounts of power can be handled by the amplifier B41.